Field of the Invention and Related Art Statement
The present invention relates to a technique for two-dimensional reduced copying of micro-patterns. The invention has been achieved mainly with the aim of supplying it to the field of solid state electronics, such as the production of very-large-scale-integrated circuits (VLSI), accompanied by micro-patterning. The invention also belongs to ultra-micro optics applicable to developing X-ray microscopes.
The increase in the degree of integration in VLSIs has brought about a decrease in the minimum line width of circuits in VLSIs. For example, the minimum line width of circuits in VLSIs is about 0.5 micron for a dynamic random access memory (DRAM) having a memory capacity of 16 M bits and about 0.3 micron for a DRAM having a capacity of 64 M bits. It is expected that a DRAM having a capacity of not less than 1 G bits will have a minimum line width of about 0.13 micron. In producing VLSIs, circuit patterns have until now been subjected to reduced copying using a system employing an optical lens. It is expected that a UV radiator such as an excimer laser will be used as a light source for copying circuit patterns having a minimum line width of not more than about 0.3 micron.
However, a pattern-reducing optical system which is substantially similar to those applied conventionally will be used even in such a case. But, when it is required to produce VLSIs having a memory capacity of not less than 1 G bits, it is expected that a pattern-copying method using X-rays has to be applied taking into account an expected minimum line width and the too long wavelength of UV light.
The refraction index of lens materials to X-rays is nearly equal to 1, and moreover, the permeability of X-rays in lens materials is extremely small. Therefore, it is impossible to produce a lens having good quality and as a result a conventional pattern-reducing copying method can not be employed.
Accordingly, it has widely been proposed to apply a conventional X-ray pattern-copying method (X-ray lithography method) which uses a mask pattern comprising a thin film 2 to 3 microns in thickness made of light elements which permit X-rays to permeate them and form a pattern thereupon using an X-ray absorber made of heavy elements such as gold, tungsten and the like. In this method, the circuit pattern drawn using the X-ray absorber is copied in its original scale on the surface of a specimen placed beneath the mask pattern by irradiating it with X-rays.
However, the above conventionally proposed X-ray lithography method as a technique for copying micro-patterns has the following problems:
1) It is not easy to produce a thin-film pattern mask.
2) It is not easy to stably maintain a thin-film mask for a long period of time because the position of the micro-pattern deviates due to the strain of the mask caused by heating during X-ray irradiation.
3) No X-ray having a wavelength of not less than several tens of A can be applied so as to avoid the absorption of X-rays by the thin-film supporting the absorber.
4) Mask adjusting is not easy because the pattern is copied in its original scale.
In order to solve the above problems 1), 2) and 3), one of the present inventors has already proposed a reflection-type pattern-copying method which, without using any thin-film pattern mask, achieves pattern-copying by irradiating a thick-plate substrate on which a pattern is drawn with beams of light such as X-ray, wherein the pattern figure is included in the reflected light (Japanese Patent Application Laid Open No. 60-173551). Furthermore, he published some of the results of studies on pattern-reflection plate formation at the 18th Conference on Solid State Devices and Materials held in Tokyo in 1986 (Matsumura, 18th Conference on Solid State Devices and Materials, Extended Abstract A-1-2, pp. 17-20). Moreover, researchers at NTT Co. have recently published a method which is based on the concept of reflection-type pattern-copying and achieves subjecting the reflected light to two-dimensional reduced copying using Schwartzschild-type convergence mirrors, thereby showing a method for solving the above mentioned problem 4).
However, the two-dimensional conventional reducing optical systems used in the X-ray region as in the case of the Schwartzschild-type mirrors, consist of a combination of two spheric mirrors. All other types, such as the Walter-type system etc. are similarly composed of a combination of at least two curved mirrors. As a result of this, new problems have arisen:
5) The structure and adjustment of a reducing optical system are complicated.
6) Decrease in X-ray intensity is profound because the X-ray is reflected many times inside the reducing optical system.